The present invention relates generally to the field of nondestructive testing. Specifically, the present invention relates to the technique of electronic shearography.
The technique of shearing interferometry, or shearography involves the interference of two laterally displaced images of the same object to form an interference image. Conventional shearographic methods require that a first interference image (or baseline image) be taken while the object is in an unstressed or first stressed condition, and another interference image be taken while the object is in a second stressed condition. Comparison of these two interference images (preferably by methods of image subtraction) reveals information about the strain concentrations and hence the integrity of the object in a single image called a shearogram. In particular, shearography has been shown to be useful to detect strain concentrations and hence defects in vehicle tires, especially retread vehicle tires.
In conventional electronic shearography, interference images are stored in a computer memory and are compared electronically to produce single static shearograms. Because all the data are processed electronically, the results of the analysis can be viewed in xe2x80x9creal timexe2x80x9d. xe2x80x9cReal timexe2x80x9d, as used in the prior art, refers to the ability to view the shearogram nearly instantaneously after the second interference image has been taken.
An apparatus and method for performing electronic shearography is described in U.S. Pat. No. 4,887,899 issued to Hung. The apparatus described in the cited patent produces an interference image by passing light, reflected from the test object, through a birefringent material and a polarizer. The birefringent material, which can be a calcite crystal splits a light ray, reflected from the object, into two rays, and the polarizer makes it possible for light rays reflected from a pair of points to interfere with each other. Thus, each point on the object generates two rays, and the result is an interference image formed by the optical interference of two laterally displaced images of the same object.
Prior to the developments disclosed in the Hung patent, the spatial frequency of the interference image produced in shearographic analysis was relatively high requiring the use of high resolution photographic film to record a useful interference image. The development disclosed in the Hung patent produces an interference image with a relatively low spatial frequency because the effective angles between the interfering rays are small. Therefore, the interference images can be recorded by a video camera, a video camera normally having much less resolving capability than a high density or high resolution photographic film. By storing an interference image of the object in its initial, unstressed condition, and by comparing that interference image, virtually instantaneously, by computer with another interference image taken under a different level of stress, a xe2x80x9creal timexe2x80x9d image or shearogram of the resultant strains on the object can be observed. Each point on the actual interference image is generated by the interference of light emanating from a pair of distinct points on the object. Therefore, each pixel of the video camera is illuminated by light reflected from those two points. If the overall illumination remains constant, then any variations in the pixel intensity, in the interference image, will be due only to changes in the phase relationship of the two points of light.
When the initial video image of the interference image is stored, an initial intensity for each pixel is recorded, as described above. If differential deformations occur in the object, such deformations will cause changes in the subsequent interference image. In particular, the intensity of a given pixel will change according to change in the phase relationship between the two rays of light, reflected from the two points on the object, which illuminate the pixel. The phase differences can be either positive changes, causing the pixel to become brighter or negative changes, causing the pixel to become darker. Whether the pixel becomes brighter or darker depends on the initial phase relationship and the direction of the change of phase. Due to the cyclic nature of phase interferences, as the deformation of the object continually increases, the intensity at a given pixel may pass through a complete cycle. That is, the intensity of the pixel might increase to a maximum (positive) difference, then return to the original intensity, and then continue to a maximum (negative) difference, and so on.
In systems of the prior art, a single shearogram is derived from two single static interference images taken at two distinct stress levels. The single shearogram is then viewed by an operator for analysis if multiple shearograms are taken, the analysis is done one shearogram at a time. Thus, the operator attendance time, required to perform a thorough stress analysis, is substantial. Further, a single shearogram may falsely show light features that appear to be defects (referred to as xe2x80x9cfalse positivesxe2x80x9d). These xe2x80x9cfalse positivesxe2x80x9d are caused by different reflective characteristics on the surface of the test object and appear as defects when a static shearogram is viewed. Further still, in a static shearogram some real defects may be xe2x80x9cwashed outxe2x80x9d and thus not visible (referred to as xe2x80x9cfalse negativesxe2x80x9d), at certain (particularly high) stress levels. These xe2x80x9cwashed outxe2x80x9d effects are caused by shearographic fringe lines that are not spatially separated enough to be visibly distinguishable and therefore appear to be aberrational light effects rather than real defects in the test object. Thus, a single static shearogram may contain inaccurate information with regards to the defects actually present. Furthermore, an operator having to analyze a large number of shearograms requires a large amount of operator attendance time.
The present invention relates to an apparatus for performing electronic shearography on a test object such as a tire. The shearography testing apparatus of the present invention can include a tire handling system which loads a tire and automatically centers it relative to the shearography camera. The tire handling system also utilizes a pivotal loading motion that eases the loading of tires and minimizes the required floor space.
The shearography testing apparatus of the present invention can also include a vacuum chamber having an air handling system which can reduce the relative humidity in the interior of the vacuum chamber during a test cycle. This can prevent the formation of a fog-like condition in the vacuum chamber which could substantially reduce the quality of the interference images taken by the shearography camera.
Another aspect of the present invention can comprise a system and method for archiving the animated images created during shearography testing so that they can be reviewed at a later time. The animated image data is compressed prior to storage on the archive medium in such a manner to allow for more efficient storage of the test results without significant degradation of the image quality. Thus, the data can be archived in a more cost efficient manner without sacrificing any loss of accuracy in the test results.
These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.